Hitherto, in order to make the life span of lubricating oil long, the following have been generally performed:    (1) a mineral type base oil is highly refined for the base oil thereof, and/or a synthetic oil such as poly-α-olefin or polyol ester is used as the base oil; and    (2) as an additive therein, there are together used a peroxide decomposer such as zinc dialkyldithiophosphate (referred to as “ZDTP” hereinafter) and a chain terminator such as a phenolic type antioxidant. In particular, ZDTP is used as an anti-oxidation and anti-wear agent, and is used as an additive indispensable to current lubricating oil, particularly, lubricating oil for internal combustion engines.
However, sulfur-containing compounds such as ZDTP cause oxidation and deterioration of lubricating oil to be accelerated with sulfuric acid discharged in the process of the oxidation or thermal decomposition of the compounds themselves although the compounds are good in antioxidation. Therefore, it has been becoming clear that there is a limit in making the lifespan of such lubricating oil longer. In particular, in compositions containing a metal-based detergent, an ashless dispersant and so on, there was a tendency that the consumption (deterioration) of the total base number thereof, which is an index of acid-neutralizing property, is accelerated. It has been therefore becoming necessary to reconsider drastically the formulation of additives, the main of which is an anti-wear agent, in order to obtain a long drain type lubricating oil very good in oxidation stability.
Separately, it is imminently necessary to moderate, as much as possible, effect on exhaust gas catalysts, such as tree-way catalysts, oxidizing catalysts and NOx absorbing and reducing catalysts, which are equipped with internal combustion engines so as to cope with the latest environmental problem, or effect on exhaust gas after-treating devices such as a DPF (diesel particulate filter). Thus, low-phosphorus, low-sulfur or low-ash lubricating oil is also desired.
The Applicant was found out that a lubricating oil composition into which a specific phosphorus-containing anti-wear agent is incorporated, as a lubricating oil in which ZDTP is decreased or is not used, is good in long drain properties, detergency at high temperature and low frictional property while the composition keeps anti-wear property, and filed a patent application about a result thereof (for example, Japanese Patent Application Nos. 2002-015351, 2001-315941 and so on). However, there is a limit in a decrease in phosphorus in order to keep the anti-wear performance of valve trains which is equivalent to that in the case where a sulfur-containing compound, such as ZDTP, is used.
Separately, investigation has been hitherto made about low-phosphorus or phosphorus-free oils. Almost all thereof are oils comprising, as an essential component, a sulfur-containing anti-wear agent such as zinc dithiocarbamate instead of ZDTP in order to keep the anti-wear performance thereof (for example, Japanese Patent Application Laid-Open (JP-A) Nos. 62-253691, 6-41568, 1-500912, 63-304095, 63-304096, 62-243692, 62-501917, and 2000-63862). Sulfur-containing compounds as disclosed therein can keep the anti-wear performance and oxidation stability to some degree in the same manner as ZDTP, as described above. It is however difficult to make the long drain properties and detergency at high temperature higher. Consequently, it has been desired to develop novel engine oil in which phosphorus or sulfur is decreased or phosphorus and sulfur are not substantially contained.